Cydia

On the left side of your iPhone above the volume buttons is a mute switch that you can use when you want to keep your iPhone's ringer from sounding off when you receive notifications such as those from phone calls, text messages, and various applications you have installed on your iOS device.

If you would find using that switch to toggle vibration more useful, then check out a new free jailbreak tweak called SwitchUp by iOS developer Aaron Thomas. With this tweak installed, flipping the mute switch one way will turn vibration on, and flipping the mute switch another way will turn vibration off.

SwitchUp adds a preferences pane to the Settings application where you can configure a couple of options:

The Inverse option will allow you to invert what way you normally flip the switch to toggle vibration on or off, and the Mode cell will allow you to choose to use the mute switch to either ONLY toggle vibration on or off (the none option), or to toggle BOTH vibration and mute (the both option).

If you would like to give SwitchUp a try, it's available for free right now in Cydia's BigBoss repository. It does require an iPhone, as the iPhone is the only iOS device that can vibrate.